Triple-Digit Heat in the Nation’s Capital ?

DC residents who don’t like extreme heat should be pleased that another June has come and gone without any triple-digit heat.  This month hasn’t featured much extreme heat as DC’s highest temperature was 93°.  That’s a little unusual since Washingtonians average one to two days of at least 95° during the month of June.

The Nation’s Capital also averages one day of triple-digit heat during June, roughly once every five years.  Meanwhile, DC residents haven’t experienced 100°/+ in June since 2012.  This June will finish as a warmer than average month with a near average number of 90° days.  High temperatures will reach 90° once again today in parts of the DC Metro Area for the ninth time this month.

Triple-digit heat is relatively rare in the Nation’s Capital in any month, typically occurring only every three to four years.  The last time Washingtonians experienced triple-digit heat was in August 2016.  It’s important to distinguish between the actual air temperature and feels-like temperature.  The “heat index” is what it feels like when you combine the air temperature with the relative humidity.  On very humid days, it feels hotter and more oppressive compared to less humid days.  The heat index exceeds the century mark several times over the course of a DC summer, but the actual air temperature rarely touches 100°.

DC’s hottest temperature on record is 106° and has occurred twice, most recently on July 20, 1930.  DC’s second hottest all-time temperature is 105° and has also occurred twice, on July 7, 2012 and August 17, 1997.  Although weather records in the Nation’s Capital date back to 1871, the more significant date to remember is 1941.  That’s when National Airport opened and where weather records have been kept since.  Prior to that, weather measurements were made downtown.  Conditions are quite different between the two locations because one has the moderating influence of the Potomac River and the other has more of an urban heat island effect.

There were some very hot summers in the 1930s prior to when National Airport opened.  However, a key difference between then and now is that urbanization has increased and overnight low temperatures remain much warmer than they used to.  That’s why the Nation’s Capital has had many of its warmest months on record over the last 10 to 20 years.  For example, June 2010 was DC’s warmest on record with the highest number of 90° days (18) while July 2012 set a new record for most 100° days with seven.  August 2016 finished with 0.1° of tying the existing record for hottest August in the Nation’s Capital.  DC’s hottest temperature in both 2017 and 2018 was 98°, while the hottest temperature in all of last year was 99°.